I had the same problem with my Samsung SCH-I760 after upgrading to WM 6.1. I went through some of the MS fixes (
https://connect.microsoft.com/feedba...634&SiteID=502 and other places on the net) and found that if I went to Options> and unchecked Tasks and then Notes, then disconnect from my # 1 PC. The reconnected and synked again I still had the same problem. I then went through this and unchecked the ones I had enabled (Contacts, Calendar, Email, Tasks and Notes). Then I enabled each one, one at a time between disconnect and re-sinking. After that it is working again between my desktop and lapton. It appears that in my case it was Task's that caused the problem.
It really shouldn't be this hard.